P-MOS control and capactive load

Thread Starter

Koxx3

Joined Sep 29, 2022
17
Hi,

I am trying to drive an high capacitive load with a P-Mosfet.
I added a PTC to limit the current, but it doesn't seen to be working. The mosfet is often killed.

Can anybody help and provide a 'simple' solution to this issue please ?

Here is the schematics.

Thanks !!

1664479665354.png
 

Ian0

Joined Aug 7, 2020
6,293
I am trying to drive an high capacitive load with a P-Mosfet.
Could you please explain in more detail what you mean by "driving" the capacitive load. Are you trying to limit the current? If so, to what value. Or are you trying to charge the capacitor as quickly as possible?
 

crutschow

Joined Mar 14, 2008
30,788
The PTC is too slow to provide any protection for this circuit.

It would appear that you are greatly exceeding the safe-area operation for Q3 (Figure 7 in the data sheet).
You could add a resistor in series with the MOSFET to reduce the peak current.

How fast do you need to charge the capacitor?

What is the input signal?
 
Last edited:

WBahn

Joined Mar 31, 2012
27,392
Hi,

I am trying to drive an high capacitive load with a P-Mosfet.
I added a PTC to limit the current, but it doesn't seen to be working. The mosfet is often killed.

Can anybody help and provide a 'simple' solution to this issue please ?

Here is the schematics.

Thanks !!
What issue? More specifically, what is it that this circuit is supposed to do and what are the constraints? How fast does the capacitor need to be charged (and what constitutes "being charged")?

How frequently is this being pulsed?

How wide is the pulse?

The transistor (at least the Vishay one I found) has an absolute max drain-source voltage rating of 100 V, so you are asking it to work right at its absolute max rating. Not good engineering.

It has a max drain current of 1 A (at room temperature) and a static on-resistance of less than an ohm, so you are initially asking it to conduct as much as 100 A briefly when the max pulsed drain current is 8 A (and it is likely well below that for the power you are asking it to dissipate during the pulse).

You might put a series resistor inline with the transistor to limit the current to something reasonable, though with 100 Ω load resistance, you can't put too much before you really start decreasing the voltage that the cap can be charged to. If necessary, you could put a second FET across this resistor that shorts it out after the capacitor voltage has risen above a certain amount.

You say that the input voltage can be "up to" 100 V. What can it be "down to"?
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
Could you please explain in more detail what you mean by "driving" the capacitive load. Are you trying to limit the current? If so, to what value. Or are you trying to charge the capacitor as quickly as possible?
yes, I'd like to limit the current. 1 to 5A max will be OK.
no need to charge quickly, it's for a power latch.
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
The PTC is too slow to provide any protection for this circuit.

It would appear that you are greatly exceeding the safe-area operation for Q3 (Figure 7 in the data sheet).
You could add a resistor in series with the MOSFET to reduce the peak current.

How fast do you need to charge the capacitor?

What is the input signal?
Yes, I understand that now.
Would an NTC IRL be better instead ?
i yes, any advise/reference to limit current to 1 to 5A ?
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
What issue? More specifically, what is it that this circuit is supposed to do and what are the constraints? How fast does the capacitor need to be charged (and what constitutes "being charged")?

How frequently is this being pulsed?

How wide is the pulse?

The transistor (at least the Vishay one I found) has an absolute max drain-source voltage rating of 100 V, so you are asking it to work right at its absolute max rating. Not good engineering.

It has a max drain current of 1 A (at room temperature) and a static on-resistance of less than an ohm, so you are initially asking it to conduct as much as 100 A briefly when the max pulsed drain current is 8 A (and it is likely well below that for the power you are asking it to dissipate during the pulse).

You might put a series resistor inline with the transistor to limit the current to something reasonable, though with 100 Ω load resistance, you can't put too much before you really start decreasing the voltage that the cap can be charged to. If necessary, you could put a second FET across this resistor that shorts it out after the capacitor voltage has risen above a certain amount.

You say that the input voltage can be "up to" 100 V. What can it be "down to"?

What issue?
>> beside dead mosfet, none ;)

More specifically, what is it that this circuit is supposed to do and what are the constraints?
>> it's a power latch for a 1-5A DC/DC converter and lights

How fast does the capacitor need to be charged (and what constitutes "being charged")?
>> no time constrain. 1s would be OK.

How frequently is this being pulsed?
>> at user power on/off

How wide is the pulse?
>> no pulse. stays on until user decide to power off.

The transistor (at least the Vishay one I found) has an absolute max drain-source voltage rating of 100 V, so you are asking it to work right at its absolute max rating. Not good engineering.
>> yes. I did expect capacotive loads at first (only lights)... but users like DC/DC for powering lights.

It has a max drain current of 1 A (at room temperature) and a static on-resistance of less than an ohm, so you are initially asking it to conduct as much as 100 A briefly when the max pulsed drain current is 8 A (and it is likely well below that for the power you are asking it to dissipate during the pulse).
>> I have ordered some HUASHUO HSU0139. datasheet.lcsc.com/lcsc/2209221830_HUASHUO-HSU0139_C701018.pdf pulse drain is at 100A, and continuous at 30A...

You might put a series resistor inline with the transistor to limit the current to something reasonable, though with 100 Ω load resistance, you can't put too much before you really start decreasing the voltage that the cap can be charged to. If necessary, you could put a second FET across this resistor that shorts it out after the capacitor voltage has risen above a certain amount.
>> yes, someone adviced me a secondary mosfet with delays. but I don't have a lot of room on the PCB.

You say that the input voltage can be "up to" 100 V. What can it be "down to"?
>> 30V to 100V

>> what do you think about replacing the PTC by an NTC to limit inrush current ?
 

LowQCab

Joined Nov 6, 2012
2,506
To design a workable Circuit requires ALL the information that You can come up with.

And, of course, You want it to fit on a Circuit-Board the size of a Quarter.

The N-FET should be replaced with a Voltage-Regulator and an Op-Amp, or similar.

1)
What does the Power-Source consist of ?
Why does it's Voltage vary ?, is it a Solar-Panel ?
What is it's absolute-maximum Voltage ?
What is it's minimum-Voltage ?

2)
Is this Circuit going inside of an Aluminum-Box that could be used for a Transistor-Heat-Sink ?

3)
What is the minimum-Voltage required to insure that the Inverter operates correctly ?
What is the maximum-Input-Voltage of the Inverter ?

4)
What Turns-On the Gate of the N-FET ?

Any other information may be important as well.
.
.
.
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
To design a workable Circuit requires ALL the information that You can come up with.

And, of course, You want it to fit on a Circuit-Board the size of a Quarter.

The N-FET should be replaced with a Voltage-Regulator and an Op-Amp, or similar.

1)
What does the Power-Source consist of ?
Why does it's Voltage vary ?, is it a Solar-Panel ?
What is it's absolute-maximum Voltage ?
What is it's minimum-Voltage ?

2)
Is this Circuit going inside of an Aluminum-Box that could be used for a Transistor-Heat-Sink ?

3)
What is the minimum-Voltage required to insure that the Inverter operates correctly ?
What is the maximum-Input-Voltage of the Inverter ?

4)
What Turns-On the Gate of the N-FET ?

Any other information may be important as well.
.
.
.
What does the Power-Source consist of ?
>> power supply is lithium battery (chosen by the end-user) with a max voltage from 10S (36V) to 22S (92V). I don't expect spikes going up to 100V

Why does it's Voltage vary ?, is it a Solar-Panel ?
>> once the battery model is chosen, it will move in the range of the battery (10S => 32V to 42V // 22S => 70V to 92V)
>> each user can have a different battery and the circuit must support those ranges

What is it's absolute-maximum Voltage ?
>> 100V

What is it's minimum-Voltage ?
>> 30V (10S empty)

Is this Circuit going inside of an Aluminum-Box that could be used for a Transistor-Heat-Sink ?
>> I can use the PCB as heatthink. case, no.

What is the minimum-Voltage required to insure that the Inverter operates correctly ?
What is the maximum-Input-Voltage of the Inverter ?
>> the output is not really an inverter, but various DC-DC designed to provide 12V for lights

What Turns-On the Gate of the N-FET ?
>> a button activated by the user

Any other information may be important as well.
>> feel free to ask, but I don't see what to add ;)
 

Ian0

Joined Aug 7, 2020
6,293
What prevents current being taken from the capacitor for the load BEFORE the capacitor has finished charging?
This is often achieved by a resistor and a relay, so that the capacitor charges through a resistor and the relay shorts out the resistor when the capacitor has charged to be close enough to the supply voltage to prevent a huge current spike.
 

Ian0

Joined Aug 7, 2020
6,293
sorry, but I don't understand.
You have a load attached to the capacitor (represented by a 100Ω resistor). What prevents this load taking current whilst the capacitor is still in the process of being charged, which you say can take as long as a second?
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
You have a load attached to the capacitor (represented by a 100Ω resistor). What prevents this load taking current whilst the capacitor is still in the process of being charged, which you say can take as long as a second?
nothing, but it's just a simulation of DC/DC with lights ON. ;)
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
What prevents current being taken from the capacitor for the load BEFORE the capacitor has finished charging?
This is often achieved by a resistor and a relay, so that the capacitor charges through a resistor and the relay shorts out the resistor when the capacitor has charged to be close enough to the supply voltage to prevent a huge current spike.
it could be an idea, but my PCB is way to small to put a relay (especially a 100V relay).
 

Thread Starter

Koxx3

Joined Sep 29, 2022
17
Its unlikely a DC/DC converter would be as large as that... have you measured the inrush current on representative DC/DC converters?
i've opened some DC/DC 100V input / 12V 10-20A ouput, there are either 100uF or 220uF.
some people use 2 of them (so 440uF total max on my circuit) ;)
 
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